This invention relates to a reel table assembly for driving a tape reel.
A tape cassette employed for use with a video tape recorder or the like generally comprises a pair of tape reels contained therein and a magnetic tape wound around the pair of tape reels from the both ends. Each tape reel has a center hole and a plurality of equally spaced and inwardly extending radial teeth formed circumferentially of the center hole.
In order to drive the tape reels, the video tape recorder has a pair of reel table assemblies. Each reel table assembly includes a disc-shaped reel table with a reel shaft formed centrally on the upper surface thereof in a body. The reel table assembly is secured on a drive shaft which is connected to a reel motor for rotating the reel table assembly.
The reel table has a friction member thereon for providing a high degree of friction with the bottom of the tape reel by directly contacting therewith so as to permit rotation of the tape reel with rotation of the reel table.
The reel table assembly has a plurality of equally spaced and radially extending drive pawls wich is mounted on the circumferential surface of the reel shaft. In order to prevent the radial teeth of the tape reel from putting on the drive pawls when the phase of each other is coincident, the drive pawls are so designed to project outwardly of the reel shaft under a resilient force for mesh engagement with the radial teeth of the tape reel, and to be movable radially inwardly of the reel shaft when the radial teeth are in radial abutment with the drive pawls.
A torque is transmitted from the reel table assembly to the tape reel through the mesh engagement between the drive pawls and the radial teeth and also through the friction between the friction member and the bottom of the tape reel. This permits certain transmission of a greatly changing torque produced on a reel motor required for high-accuracy servo control of the tension on the magnetic tape.
However, when the tape cassette is inserted in the cassette holder of the video tape recorder and the center hole of the tape reel receives the reel shaft, the drive pawls are often pushed radially inwardly of the reel shaft, as the radial teeth of the tape reel are in radial abutment on the drive pawls.
The friction between the friction member and the tape reel is too great to allow smooth sliding movement of the tape reel with respect to the reel table with the result that the drive pawls will remain in radial abutment with the radial teeth. This is true particularly when the tape reel has a large diameter and carries a great number of turns of magnetic tape thereon. Under this condition, the torque transmission is made from the reel table to the tape reel only through the friction between the friction member and the tape reel, resulting in an excessive transient tension on the running tape. If the torque transmitted to the tape reel exceeds a level that can be transmitted through the friction between the friction member and the tape reel, slip will occur between the reel table and the tape reel to decrease the accuracy of the tape tension servo control.